outdrive rebuild

I’m restoring a 18’ 1970 Sea Rey Pachanga with a 327 Chevy / and I assume a 1st gen Merc cruiser outdrive. I understand that the thing to do is get rid of the outdrive and replace it with a newer Merc cruiser outdrive?? My questions are. What outdrive to use and will I need to glass up the...

If anyone can offer some advice I would really appreciate it. I have a 23' 1995 sea ray sundancer with a 5.0 merc/ alpha one second gen outdrive. stainless rapture prop Problem: I have no thrust in higher rpm's and I am not able to get on plane Possible reasons: 1) I keep the boat in the bay in...